1. Field of the Invention
This invention relates to light measuring devices for single lens reflex cameras, and more particularly to light measuring devices of the type in which light from the focusing screen is received through the optical path bending prism arranged in the viewfinder.
2. Description of the Prior Art
The light measuring has been performed by putting an image forming lens and a light receiving element behind the exit face of the penta dach prism. The conventional light measuring devices of this kind have features that the structure is simple, and the space the light measuring device occupies is relatively readily available. But, because the image forming lens (collection lens) must be positioned away from the optical axis of the finder so as not to mutilate the path of light to an eye observing the finder, there was a drawback that the relationship of the F-number of the objective lens and the amount of light measured, or the so-called F-number proportionality was not good.
This reason is next explained by using drawings. FIG. 1 schematically illustrates the single lens reflex camera having the conventional light measuring device of this kind, where a focusing screen 1, a pentagonal roof type prism 2, an image forming lens 3, a light receiving element 4, an eye-piece 5, a photographic lens 6 having a diaphragm therein, a rotatable mirror 7 and a photographic film 8 are included.
FIG. 2 is an expanded view illustrating an optical path to the light measuring device and another optical path to the eye-piece, where O denotes the optical axis of the finder, K the oblique pencil of rays to the eye-piece, and M the optical path from the central portion of the area of the focusing screen to the light measuring device. The other reference characters denote the similar parts to those shown in FIG. 1. The pentagonal roof type prism 2 is depicted as expanded along the optical path. As is obvious in this drawing, the image forming lens 3 and the light receiving element 4 must be arranged at a considerably far position from the optical axis O of the finder in order to insure that the optical path K to the eye-piece of the finder is not mutilated at all. As the diaphragm stops down, therefore, that part of the light from the objective lens 6 which is shown by hatching does not arrive at the light receiving element 4. This gives rise to a problem that for dark F-numbers, the amount of light measured largely deviates from the proportion to the F-number of the photographic lens 6, as shown in FIG. 3.
In the graph of FIG. 3, the abscissa is in the F-number of the objective lens 6 and the ordinate is in the number of steps E of the measured light amount with reference to that when F/1.4 (where E=log.sub.2 Q'-log.sub.2 Q wherein Q is the measured light amount at F/1.4 and Q' is the measured light amount at any other value of the F-numer). A straight line labelled I represents that the measured light amount is strictly proportional to the F-number, or the ideal F-number proportionality. The conventional light measuring device has an F-number proportionality shown by curve R. As has been stated above, it is in dark F-numbers that the F-number and the measured light amount become unproportional to each other. For note, the cause of breaking the F-number proportionality even at the brightest ones of the F-numbers is that the aperture opening of the light measuring optical system cannot be made sufficiently large, so that the light at the bright F-number does not fully enter the light receiving element 4. This problem is solved usually by using a signal pin on the photographic lens mounting in sensing the F-number at the full open aperture of the photographic lens 6 for the purpose of correcting the measured light amount.
Though the conventional light measuring devices of this kind have the above-described problem, it is actually due to the diffusing action of the matted surface D of the focusing screen as shown in FIG. 4 that even for the dark F-numbers, the diffusing light is allowed to enter the light receiving element 4 through the image forming lens 3, thus moderating the above-described problem. However, the generally accepted focusing screens 1 have a range finder area S at the center thereof in the form of split prisms as shown in FIG. 5. Because this range finder area is transparent, if the light measuring is carried out by the light only from this area, the above-described problem is intensified. That is, as the light measuring area narrows, when the ratio of the range finder to the light measuring area approaches unity, the F-number proportionarity is deteriorated badly.
For this reason, the use of a partial field of view light measuring mode or spot measuring mode worsens the F-number proportionality. As a result, when the diaphragm of the photographic lens 6 stops down, it becomes impossible to perform accurate light measuring. Also when the photographic lens 6 is interchanged, the amount of light measured must be corrected. Further in the camera of which the focusing screen is interchangeable, another problem is produced that the correction of the measured light amount depends on the form of the range finding area S. Also, in the light measuring device operating with selection of light measuring areas of different size, as the size of the light measuring area changes, another problem arises that the F-number proportionality becomes uncertain.